Coupled optical waveguide resonators with heaters for thermo-optic control of wavelength and compound filter shape

ABSTRACT

An integrated optical device is disclosed comprising a substrate, optical waveguide, and compound optical resonator having a temperature sensor, at least two coupled optical resonators, and a heater localized to each optical resonator. An optical input signal is coupled to one of the resonators making up the compound resonator to form an optical output signal. The center wavelength and shape of the output signal is optimized with a feedback loop using the temperature sensor to control the power dissipated in at least one of the localized heaters. The power dissipated in the remaining resonator heaters is set according to a predetermined function having as an input variable the power dissipated in the resonant heater under control of the said feedback loop.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. Ser. No. 11/800,591filed on May 7, 2007, now U.S. Pat. No. 7,356,221 which is acontinuation of U.S. Ser. No. 11/208,338 filed on Aug. 19, 2005, nowU.S. Pat. No. 7,231,113 the entire content of both incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to integrated optical circuits having opticalwaveguides wherein a portion of the optical waveguides' refractive indexis adjusted thermo-optically. More particularly, this invention pertainsto thermo-optic control of the transfer function of a compound opticalresonator by use of individual heaters localized to each resonatorwaveguide element and a local temperature sensor.

2. Description of the Related Art

The field of integrated optics has expanded tremendously over the pastdecade, and integrated optical device solutions are now being proposedfor applications in a variety of fields including: telecommunications,data communications, high performance computing, biological and chemicalsensing, and RF networks. Long haul, metropolitan, and local networks,as well as fiber-to-the-home applications, predominately rely on opticalcommunications for data transmission. Wavelength division multiplexing(WDM) technologies have enabled a dramatic increase in thecommunications bandwidth of optical fiber. At channel spacings of 50GHz, hundreds of signal channels are possible within the S-, C-, andL-bands of optical fiber covering the wavelength region from 1460 to1625 nm. Some fiber-to-the-home applications utilize the O-band(1260-1360 nm) as well. More recently, optical data transmission hasbeen investigated for computing systems to enable high bandwidthcommunications between processors, memory, and I/O. Improved RF systemdesigns, such as phased array radars, with integrated optical devices inplace of conventional microwave components have been shown to havehigher potential bandwidths and target resolution. Lastly, spectroscopicapplications of integrated optical devices in the ultra-violet andvisible wavelength regions are being considered for biological andchemical sensors.

Integrated optics is the technology of integrating various opticaldevices and components to transport, focus, multiplex, demultiplex,split, combine, polarize, isolate, couple, switch, filter, modulate(phase or amplitude), detect, and generate light. Integrated opticaldevices combine several of these functions on a common chip orsubstrate. Examples include commercial planar lightwave circuits (PLCs),such as those that have been installed in WDM communication systems tomultiplex and demultiplex optical channels on a fiber. More complexmultiple layer PLCs have also been designed that incorporate waveguidesand device circuitry on multiple planar layers interconnected usingoptical via technology. Newer integrated devices such as photonicintegrated circuits (PICs) are now being developed by companies such asInfinera (Sunnyvale, Calif.) for high performance network operatingsystems and optical routing and switching applications.

Optical waveguides are the key building block of integrated opticaldevices. Optical waveguides are light conduits consisting of a slab,strip, or cylinder of dielectric core material surrounded by dielectriccladding materials of lower refractive index. FIG. 1 depicts an exampleplanar optical waveguide 40, known in the art as a ridge waveguide,formed on a substrate or wafer 42 by the formation of a lower opticalcladding 44; chemical vapor deposition, lithographic patterning, andetching of an optical core element 46; and lastly by surrounding theoptical core element with an upper optical cladding layer 48. Othertypes of optical waveguide designs include rib, trench, filled trench,and strip-loaded waveguides. Typical lateral and vertical dimensions ofthe core elements in glass-based optical planar lightwave circuitwaveguides lie between about 0.5 and 5 microns.

An optical waveguide or combination of optical waveguides can beassembled to form devices such as: optical resonators, arrayed waveguidegratings, couplers, splitters, polarization splitters/combiners,polarization rotators, mach-zehnder interferometers, multimodeinterference waveguides, gratings, mode transformers, delay lines, andoptical vias. Devices such as these may then be combined or integratedon an optical chip to create an integrated optical device or planarlightwave circuit that performs one or more optical functions such as:multiplexing/demultiplexing, optical add/drop, variable attenuation,switching, splitting/combining, filtering, spectral analysis, variableoptical delay, clock distribution, amplitude/phase modulation,polarization rotation, comb generation, and dispersion compensation.

Integrated optical devices and planar lightwave circuits can befabricated on a variety of substrates or wafers. Some of the more commonmaterials used are silicon, silicon wafers having silicon-oxide (SiO₂)or thermal oxide layer coatings, and indium-phosphide (InP). Othermaterials considered for substrate or wafer applications includegermanium, silica, fused quartz, sapphire, alumina, glass,gallium-arsenide, silicon-carbide, lithium-niobate,silicon-on-insulator, germanium-on-insulator, and silicon-germanium.Integrated optical devices or planar lightwave circuits may also befabricated on or placed over preformed devices or circuits such as: oneor more electrical devices (e.g. transistors), optical devices (e.g.mode transformers), microelectromechanical (MEMS) devices (e.g.mirrors), or optoelectronic devices (e.g. detectors, amplifiers,modulators, light emitting diodes, or lasers).

The material system most commonly used for planar optical waveguidedevices is germanium doped silicon oxide SiO₂:Ge. The waveguide consistsof a SiO₂:Ge optical core element, having a refractive index, that issurrounded by lower and upper cladding layers having smaller refractiveindices. Typical cladding layer materials include air, polymer, silica(SiO₂) and doped silicas such as phosphosilicate glass (PSG),borosilicate glass (BSG), borophosphosilicate glass (BPSG), and flourinedoped silica (SiOF). The ability to shrink the dimensions of an opticalcircuit is generally limited by the refractive index contrast of theoptical waveguides from which it is formed, where the refractive indexcontrast is defined as the ratio (n_(core)−n_(clad))/n_(clad). Thesmallest possible size of an optical device is constrained by theminimum allowable radius of curvature of its optical waveguides beforeincurring significant optical propagation loss of 0.5 dB/cm or more.Lower optical losses of less than 0.2 dB/cm are preferred. The higherthe index contrast, the easier it is for waveguides to be routed on achip using tight turns and small bend radii of curvature. For SiO₂:Gebased optical waveguides, the maximum index contrast is limited to about0.03 (3%), resulting in a minimum radius of curvature of at least 500mm.

Silicon-oxynitride (SiON) is another doped silica that has been used forthe fabrication of planar lightwave circuits (see, e.g., G. L. Bona, R.Germann, and B. J. Offrein, “SiON high refractive index waveguide andplanar lightwave circuits,” IBM J. Res. & Dev. 47 (2/3), 239 (2003)incorporated herein by reference). SiON materials can be formed that aresubstantially transparent with an absorption loss of less than about 0.5dB/cm in the wavelength region from 700 nm to 1650 nm. For thewavelength range from 1460 nm to 1625 nm, propagation losses of lessthan 0.2 dB/cm have been demonstrated. Low loss optical waveguides havebeen demonstrated having SiON core elements and SiO₂ cladding layerswith significantly higher refractive index contrasts than is possiblewith SiO₂:Ge (see, e.g., U.S. Pat. No. 6,614,977). Optical waveguideswith refractive index contrasts of about 0.17 (17%) can be fabricatedfrom SiON core elements, having a refractive index of 1.7, and SiO₂cladding layers, having a refractive index of about 1.45 measured at awavelength of 1550 nm. At this high index contrast, waveguides can bedesigned with radii of curvature as small as about 35 mm. For the SiONcore elements, the highest potential refractive index value is 2,corresponding to silicon-nitride (Si₃N₄). Other materials being lookedat for high index contrast waveguide applications include silicon,germanium, indium-phosphide, aluminum-oxide, silicon-oxycarbide (SiOC),and polymers.

A common optical element in most optical integrated circuits is athermo-optic device, wherein a portion of the optical core waveguidingelements within the device is locally heated with respect to rest of thedevice by use of a local resistive heating element. Local heating of awaveguide shifts the phase of an optical signal within the waveguide byway of the thermo-optic effect on refractive index and is usuallydenoted by the change of refractive index with change in temperature ordn/dT. In addition, there can be a secondary contribution to thethermally induced phase shift from thermal expansion of waveguidedimensions. For example, an optical device such as a ring resonator hasa set of resonant wavelengths given approximately by:l _(i)=2prn/iwhere r is the ring radius, n is the optical waveguide effective index,and i is an integer. The resonant wavelengths of the ring may be changedby locally raising or lowering the temperature of the ring waveguide,and the effective refractive index may be approximated byn=n _(o)+(dn/dT)DTwhere n_(o) is the effective index at the initial temperature T_(o),dn/dT is the rate of change of refractive index with temperature atT_(o), and DT is the net change in temperature T−T_(o). For opticaldevices formed from SiO₂:Ge, the dn/dT is typically just under 1×10⁻⁵°C. For optical devices formed from chemical vapor deposited SiON-basedoptical waveguides, the dn/dT is typically about 1.1 to 1.4×10⁻⁵/° C.depending on the composition. Applications of optical resonatorthermo-optic devices requiring precise control of resonant frequency ofbetter than ±1 GHz are not well suited to materials with high dn/dTmagnitudes greater than about 5×10⁻⁵. A large dn/dT in this case is notdesirable as it will require very precise local temperature control onthe order of 0.1° C. Low dn/dT magnitudes of less than about 1×10⁻⁵,however, require somewhat more power as larger local temperature changesare needed to achieve a desired refractive index change, Dn.

Optical resonators are optical cavities supporting standing or travelingwave resonant optical modes. This invention considers an opticalresonator to be any closed loop optical waveguide or disk supportingresonant optical modes. Examples of optical resonator waveguides 60 aredepicted in FIG. 2 and include an optical waveguide core with a diskgeometry 62, ring geometry 64, ellipse geometry 66, or racetrackgeometry 68. Optical resonator waveguides 60 having the ring geometry 64are the most commonly studied resonator cavity for integrated opticaldevices with a set of resonant wavelengths determined by the radius andeffective index of the cavity. Optical resonators can be used singly orin multiples to form a compound optical resonator. Higher order cascadesof optically coupled resonators are depicted in FIG. 3 showing second80, third 82, and fifth 84 order compound optical ring resonatorcavities. The Lorentzian response of a single-ring, channel droppingfilter can be improved upon to achieve a wider and flatter passband aswell as larger out of band signal rejection by moving to higher orderfilters, that is compound resonators consisting of multiple opticallycoupled resonators. The theory of compound optical resonators isdiscussed by B. E. Little et al. in “Microring resonator channeldropping filters,” Journal of Lightwave Technology, Vol. 15, No. 6, pp998-1005 (1997) and is incorporated herein by reference.

Compound optical resonators must be optically coupled to at least oneoptical waveguide to form a useful optical device. A compound opticalresonator cavity coupled to a single optical waveguide, as shown by 90in FIG. 4, can be used to modify the phase of an optical signal in anall-pass filter or a dispersion compensator device. A compound opticalresonator cavity coupled to two optical waveguides, depicted by 92 inFIG. 4, can be used as a channel dropping filter in which an opticalsignal or set of optical signals resonant with the modes of the compoundoptical resonator are dropped from one optical waveguide and added tothe other. In these examples, the optical waveguide or waveguides mustbe adjacent to one of the resonator waveguides of the compound opticalresonator such that the optical modes are coupled. In FIG. 4, opticalwaveguides are shown that are positioned laterally adjacent 92, above94, and below 96 optical resonators.

The transfer function of an optical system is the effect on an opticalsignal in the frequency domain. The transfer function, for example, ofan optical filter device acting on an optical input signal determinesboth the shape and center wavelength (frequency) of the optical output.A filter composed of a single ring resonator will take an optically flatinput signal and transform it into a set of Lorentzian shaped outputsignals centered at the resonant wavelengths of the resonator. Opticalfilters composed of multiple and identical coupled ring resonatorsproduce an output signal having a flatter and broader passband as wellas larger out of band signal rejection.

The output of a thermo-optic compound optical resonator device willgenerally remain at a fixed wavelength unless there is a change intemperature. The temperature may be changed globally using a heater orPeltier element such as a thermoelectric cooler (TEC) that is in thermalcontact with the integrated optical device substrate. It is moredesirable, however, to use a heater or set of individual heaterslocalized to each compound thermo-optic element in the integratedoptical device to allow for independent control. Simple resistiveheaters can be fabricated by the deposition and patterning of metalfilms (such as platinum, gold, aluminum, chrome, nickel, nichrome, ortungsten) or semiconductor materials such as polysilicon. Other possibleheater designs include localized Peltier elements.

Some commercially available thermo-optic devices such as switch arraysand optical attentuators operate by the simple application of apredetermined fixed power to each thermo-optic element heater in orderto achieve a calibrated optical state. More sophisticated devices, suchas those incorporating compound resonators, require a method of controlthat utilizes a feedback control loop. One possible feedback method isto analyze the optical output signal itself, by monitoring the signal'scenter wavelength for example, and use this parameter to determine theappropriate heater power dissipation applied to the thermo-optic elementin real time. This method becomes unwieldy for a complex integratedoptical device and would involve multiple real time optical measurementsfor the elements within the device. A much simpler approach is to use asa feedback variable for each heater a measurement of the localtemperature. The most simple device used for temperature measurement arethermistors or resistive temperature devices (RTDs) that have aresistance value that changes as a function of the local temperature.For example, a metal film resistor can be deposited around or over anoptical resonator waveguide in order to monitor the local averagetemperature. Platinum is the most common metal used to fabricate metalfilm RTDs, but copper, nickel, and nickel-iron are also used.Thermistors are generally made from semiconducting metal-oxide ceramics.Other example temperature sensors include a temperature sensing diode ora thermocouple, which generates an electric potential between twodissimilar metals that is a function of temperature

A control system is required to adjust, regulate, or control the powerdissipation in a heater. Generally, a control system will monitor thefeedback variable from a temperature sensor (e.g. the resistance valueof an RTD) and change the power dissipated in the heater as needed tobring the feedback variable to a predetermined set value or setpoint.Example controllers include simple on/off, proportional, proportionalbandwidth, and PID type controllers. Examples of these controllers arecommercially available from vendors such as Omega Engineering Inc.,Stamford Conn. Integrated optical devices, however, generally integratethe control functions and electronic devices within a printed circuitboard. The controllers can adjust the heater power dissipation throughan analog change in voltage (current) applied to a heater or through achange in the duty cycle of a digital pulse train, or filtered digitalpulse train, known as pulse width modulation (PWM). PWM controllers arecommercially available as digital signal processor chips from companiessuch as Freescale Semiconductor Inc., Austin Tex. (e.g. DSP56F8XXseries). Often these functions are integrated into a printed circuitboard as well.

Prior art work has focused on methods of thermally tuning opticalresonators composed of a single ring. In P. Heimala et al., “Thermallytunable integrated optical ring resonator with poly-si thermistor,”Journal of Lightwave Technology, Vol. 14, No. 10, pp 2260-2267 (1996),incorporated herein by reference, the authors disclose a Si₃N₄rib-waveguide-based ring resonator integrated with a local thermistorand polysilicon heater. In this paper, the resonant wavelength isadjusted using the feedback of a thermistor to regulate the powerdissipated in a heater overlaying a ring waveguide. U.S. Pat. No.6,636,668, incorporated herein by reference, also discloses a thermallytunable resonator device, each resonator comprising a heater and atemperature sensor. Compound resonators comprising multiple rings arediscussed in U.S. Pat. No. 6,411,752, incorporated herein by reference,where the use of localized heaters placed above or below each ring isproposed as a method of tuning, however, there is no mention of feedbackcontrol methods.

Although it is relatively straightforward to control the resonantwavelength of a single ring resonator using a heater and a temperaturesensor, compound optical resonators comprising at least two opticallycoupled resonators are more complex. One could simply devise a designincorporating one large area heater that covered all the opticalresonators of the compound resonator. The resonant wavelength of thecompound resonator could then be adjusted using feedback from a singletemperature sensor to control the power sent to the one heater. Thereare two major limitations of this design. One, a large area heater willuse significantly more power than an individual heater that issubstantially of similar size and conformal in shape to the opticalresonator waveguides. Two, it is difficult to devise a large area heaterthat produces a laterally uniform temperature profile across a set ofcoupled resonators and over a reasonably large temperature range of manytens of degrees Celsius. This type of heater will generally be hotter inthe center and cooler near the edges resulting in an offset in resonantwavelength, or detuning, between optical resonators positioned nearerthe center and nearer the edge of the heater. The result would be adeterioration in the compound filter shape from optimum.

An alternate method would be simply to use independent control for eachresonator in the compound resonator as disclosed by Kuipers et. al. in“Integrated optical signal handling device,” International PublicationNo. WO 02/103448 A2. There are several major disadvantages with thismethod. First, each optical resonator within the integrated opticaldevice would need its own heater and temperature sensor. This greatlyincreases the number of electrical connections required by the opticalchip. In addition, a complete feedback loop is required for eachindividual heater with electronics and software dedicated to themonitoring of each temperature sensor over time. An integrated opticalchip having four compound resonators each consisting of five rings wouldrequire twenty temperature sensors and feedback loops. Second,independent feedback control loops localized to each ring will beinfluenced by the control loop operating on adjacent optical resonators.The optical coupling requirement between resonators causes a portion ofthe outer surfaces of each resonator waveguide to be positioned withinabout a micron or less of each other. This separation is known as thecoupling gap. The small separation means that there will be significantthermal crosstalk between the temperature sensor of one resonator andthe heaters of adjacent resonators that results in control loopinstability.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the features and advantages of the present invention can beunderstood in detail, a particular description of the invention may behad by reference to the embodiments thereof that are illustrated in theappended drawings. It is to be noted, however, that the appendeddrawings illustrate only typical embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

FIG. 1. Diagram of a ridge waveguide.

FIG. 2. Diagrams of disk, ring, elliptical, and racetrack opticalresonator geometries.

FIG. 3. Diagram of compound optical ring resonators having two, three,and five optically coupled rings.

FIG. 4 Diagram of compound optical resonators optically coupled to oneor two adjacent optical waveguides that reside beside, above, or below aring resonator.

FIG. 5 Diagram of the metal pattern for the heaters and RTD associatedwith a compound optical resonator having three ring resonators.

FIG. 6 Cross-sectional view depiction down the center line of thecompound resonator structure of FIG. 5 showing heaters, RTD, optical andring resonator waveguides.

FIG. 7 Block diagram layout of an example circuit controlling a 3-ringcompound optical resonator.

FIG. 8 Data showing the optical transfer function response for anoptical signal traversing a 3-ring compound optical resonator with theheaters-off and with the heaters-on, where the heaters-on powerdissipation is optimized to achieve a flat response.

FIG. 9 Data showing the optical transfer function response for anoptical signal traversing a 3-ring compound optical resonator for sevendifferent optimized heater power dissipation settings.

FIG. 10 Example data showing functional relationship between the optimumpower dissipation in an outer ring of a 3-ring compound resonator versusthe power dissipation in the center ring, which is under control of thefeedback loop.

DETAILED DESCRIPTION

Presently preferred embodiments of the invention are shown in theabove-identified figures and described in detail below. In describingthe preferred embodiments, like or identical reference numerals are usedto identify common or similar elements. That is, the reference numeralsinclude similar numeric prefixes and different alphabetic suffixes tospecifically identify particular elements in the drawings. The numericprefix by itself refers to a group of elements and not to any particularelement. The figures are not necessarily to scale and certain featuresand certain views of the figures may be shown exaggerated in scale or inschematic in the interest of clarity and conciseness.

Referring now to the drawings, and more particularly to FIGS. 5 and 6,shown therein is an integrated optical device 10 constructed inaccordance with the present invention. The integrated optical device 10is provided with a substrate 12, at least one optical waveguide 14 (twoare shown and labeled with reference numerals 14 a and 14 b), at leastone compound optical resonator 16 (comprising coupled optical resonators30 a, 30 b, and 30 c), first (32 a and 32 c) and second (32 b) subsetsof heaters, and one or more temperature sensor 22. The at least oneoptical waveguide 14 a or 14 b resides on the substrate 12. The at leastone compound optical resonator 16 is optically coupled with the at leastone waveguide 14 a or 14 b. The compound optical resonator 16 includesat least two coupled optical resonators shown and labeled with thereference numerals 30 a, 30 b, and 30 c. Each of the heaters making upthe first and second subsets are shown and labeled with referencenumerals 32 a, 32 b, 32 c where each is localized to at least one of theoptical resonators 30 a, 30 b or 30 c of the compound optical resonator16. The temperature sensor 22 is localized to at least two of thecoupled optical resonators 30 a, 30 b, and 30 c of said compound opticalresonator 16.

FIGS. 1-4 will now be described in more detail. As discussed above inthe background section, FIGS. 1-4 illustrate prior art components orprocedures which are utilized to construct portions of the integratedoptical device 10. More particularly, FIG. 1 illustrates across-sectional view of an exemplary planar optical waveguide 40 knownin the art as a ridge. The planar optical ridge waveguide 40 is formedon a substrate or wafer 42 by the formation of a lower optical cladding44 on the substrate 42 and an optical core element 46 formed on thelower optical cladding 44 using any suitable technique, such as chemicalvapor deposition, such that the optical core element 46 also resides onthe substrate 42. The optical core element 46 is then shaped orpatterned using, for example, lithographic patterning and etching of theoptical core element 46. Then, the shaped optical core element 46 issurrounded with an upper optical cladding layer 48 (see e.g. U.S. patentapplication Ser. No. 10/864,511). The optical core element 46 forms anoptical waveguide that is a fundamental building block for makingoptical resonators 60, compound optical resonators 16, and integratedoptical devices 10. Planar optical waveguides 40 can be used to form asingle optical device (e.g., an individual microresonator) or an opticalsystem (e.g., a series or cascade of microresonators formed into ademultiplexer or tunable filter). The procedure discussed above forforming the planar optical waveguide 40 can be used for forming theoptical resonators 60 and waveguide(s) 14 of the integrated opticaldevice 10.

Turning again to FIG. 1, the substrate 42 is selected from any suitablematerial or wafer such as silicon. The substrate 42 may be constructedof other materials and may include electrical, optical, andoptoelectronic devices and circuits. The substrate or wafer 42 mayconsist of a single material (e.g. a silicon wafer), a stack of severaldifferent materials (e.g. a layer of thermal oxide (SiO₂) on silicon),or a preformed device or circuit. A preformed device or circuit maycontain one or more electrical devices (e.g. transistors), opticaldevices (e.g. mode transformers), microelectromechanical systems (MEMS)devices (e.g. mirrors), and optoelectronic devices (e.g. detectors,amplifiers, modulators, light emitting diodes, and lasers).

The compound optical resonator 16 is formed from the combination of atleast two optical resonators 30 a, 30 b, and 30 c. FIG. 2 depictsseveral examples of optical resonator waveguides 60 having a disk 62,ring 64, ellipse 66, or racetrack 68 geometry. However, other closedloop geometries are feasible as well. The optical resonators 30 a, 30 b,and 30 c making up the compound optical resonator 16 are configured suchthat the closest distance, or coupling gaps 108 a and 108 b, between theouter surfaces of one optical resonator 30 a, 30 b, or 30 c and anadjacent optical resonator 30 a, 30 b, or 30 c allows for opticalcoupling of a portion or all of the signal from one optical resonator 30a, 30 b, or 30 c to the adjacent optical resonator 30 a, 30 b, or 30 c.FIG. 3 shows example configurations of the compound optical resonator 16designated by the reference numerals 80, 82, and 84. The compoundoptical resonator 80 includes two of the optical resonators 64 a and 64b. The compound optical resonator 82 includes three of the opticalresonators 64 c, 64 d, and 64 e. The compound optical resonator 84includes five of the optical resonators 64 f, 64 g, 64 h, 64 i and 64 j.

The combined unit of optical resonators 30 a, 30 b, and 30 c forming thecompound optical resonator 16 of the integrated optical device 10 mustalso be optically coupled to at least one of the optical waveguide(s) 14a or 14 b bearing an optical signal. FIG. 4 shows example configurations(identified by reference numerals 90, 92, 94 and 96) of the compoundoptical resonator 16 optically coupled to various configurations ofoptical waveguide(s) 14 via one or more coupling gap(s) 98. Inparticular, the example configuration identified by reference numeral 90includes a single, laterally adjacent optical waveguide 100 a opticallycoupled with the compound optical resonator 82 a via a coupling gap 98a. In this configuration, the input optical signal enters and the outputoptical signal exits the compound optical resonator 82 a via the sameoptical waveguide 100 a. This type of configuration is sometimes knownas an all-pass filter and can be used, for example, to modulate thephase of an optical signal such as in a dispersion compensator. A moreconventional design that is used for applications such as add/dropmultiplexing and tunable filtering is designated by the referencenumeral 92 in FIG. 4. Here, two optical waveguides 100 b and 100 c(e.g., referred to below as upper and lower waveguides) are positionednear the compound optical resonator 82 b laterally adjacent to the twoouter optical resonators 64 c and 64 e of the three-optical resonatorcompound optical resonator 82 b. In this case, an optical signaltraversing the upper waveguide 100 b that is resonant with one of theoptical modes of the compound optical resonator 82 b will traverse theresonator cavity and exit via the lower waveguide. The arrangementsidentified by the reference numerals 94 and 96 are similar to thearrangement identified by the reference numeral 92 except that in thearrangement 94 the waveguides 100 d and 100 e are positioned above theoptical resonators 64, and in the arrangement 96 the waveguides 100 fand 100 g are positioned below the optical resonators 64. The integratedoptical device 10 can utilize any combination of optical resonators,(e.g., 62, 64, 66 and 68 depicted in FIG. 2), number of opticalresonators (e.g. 80, 82, and 84 depicted in FIG. 3), and arrangement ofone or more waveguides (e.g. 90, 92, 94, and 96 depicted in FIG. 4).

As shown in FIG. 5, the integrated optical device 10 of the presentinvention has individual heaters 32 a, 32 b, and 32 c localized to eachoptical resonator 30 a, 30 b, and 30 c in the compound resonator 16. Theheaters are labeled in FIG. 5 with the reference numerals 32 a, 32 b and32 c for purposes of clarity. Each of the heaters 32 a, 32 b, and 32 cinclude a heater element 110 a, 110 b, and 110 c and a pair of leads 112a, 112 b, 112 c, 114 a, 114 b, and 114 c for supplying power to theheater elements 110 a, 110 b, and 110 c. To minimize both powerconsumption and the intrinsic thermal time constant of compoundresonator elements 16 within a thermo-optic device, it is preferred tolimit the volume of heated material to the core element region of theoptical resonators 30 a, 30 b, and 30 c. Thus, it is preferred that theheater elements 110 a, 110 b, and 110 c be substantially conformal withand localized to the optical resonator waveguides. In general, theheater elements 110 a, 110 b, and 110 c will have a length that followsthe path of the optical resonator waveguide and a width that isdesirably between one to ten times the width of the optical resonatorwaveguide 30 a, 30 b, and 30 c core elements. The heaters are individualin that the power dissipation of each heater 32 a, 32 b, and 32 c may beset independently of the other heaters in the integrated optical device10. This requires that at least one lead 112 a, 112 b, 112 c, and 114 a,114 b, and 114 c of the heater be independent, while the second lead 112a, 112 b, and 112 c, may be independent or electrically connected withother leads 112 a, 112 b, 112 c, 114 a, 114 b, and 114 c to a commonvoltage or ground.

As discussed above, the integrated optical device 10 of the presentinvention provides the temperature sensor 22 localized to at least twoof the optical resonators 30 a, 30 b, and 30 c and preferably all of theoptical resonators 30 a, 30 b, and 30 c. It is preferred to utilize onlyone temperature sensor 22 as the feedback element for each compoundresonator 16. The temperature sensor 22 may be a resistive temperaturedevice (RTD) such as a metal film resistor, a thermistor, thermocouple,or temperature sensing diode. For an example case of an RTD orthermistor, it is preferred that the trace path of the film lie in closeproximity to the optical core element but not so close as to induceoptical loss. As an example, an RTD may lie in a plane near the opticalcore waveguide elements or trace a path adjacent to the set ofindividual heaters 32 a, 32 b, and 32 c. In either case, the measuredtemperature sensor value will be a resistance substantiallyrepresentative of the average temperature along its path. For RTDs, itis desirable to choose a metal film deposition thickness, trace width,and length to achieve a resistance value between about 100 and 10000ohms. Thermocouples and temperature sensing diodes can be localized moreto a point location near to each optical resonator 30 a, 30 b, and 30 c.

As an example, FIG. 5 shows the individual heaters 32 a, 32 b, and 32 cand temperature sensor 22 combination of a three-ring compound opticalresonator 82 having ring radii of about 45 microns. The heaters can beformed via any suitable process. For example, in one preferredembodiment, a 0.2 micron thick platinum metal film is deposited usingelectron-beam thermal deposition. The metal is deposited over aphoto-lithographically patterned layer of photoresist having openwindows substantially over the optical resonator waveguides as depicted.After a lift-off procedure, platinum remains in the open window areas.With an example width of 8 microns, the heater resistance isapproximately 25 ohms. Generally, however, heater resistances of between10 and 500 ohms are acceptable. In this example, the heater trace leads112 a, 112 b,112 c, 114 a, 114 b, and 114 c broaden and connect to goldstrip lines (not shown) leading to a bond pad. From the bond pad, thetrace can be connected through a wire bond, or other packagingtechnique, to a chip package and ultimately a printed circuit board.

FIG. 5 also shows the path traced by a single platinum RTD temperaturesensing element 22. In this example, the temperature sensing element 22is coplanar with the heaters 32 a, 32 b, and 32 c and located near andadjacent. The platinum metal film is formed in a similar manner to theplatinum heater discussed above. The deposition thickness is about 0.2microns, and the trace width and spacing are 2 microns such that theresistance is about 800 ohms at 300 K. The technology of platinumresistance thermometry is well known in the field.

FIG. 6 contains a cross-sectional view down the center line of theexample compound resonator structure of FIG. 5. The layout of theheaters 32 a, 32 b, and 32 c and RTD 22 trace cross-sections are shownin the top layer. These metal traces may be air clad or encapsulated ina protective overcladding material such as SiO₂. Just below is an SiO₂optical cladding layer 102 (see eg. U.S. Pat. No. 6,768,828) in whichthe microring resonator waveguides 30 and optical waveguides 14 lie. Inthis example, the lateral and vertical waveguide dimensions are about1.5 microns, the waveguide to resonator coupling gaps 98 are about 0.3microns, and the resonator to resonator coupling gaps 108 are about 0.6microns. Coupling gaps 98 and 108 are design dependent, but separationsgenerally lie between about 0.2 to 2 microns. Lastly, a lower claddingmaterial layer 106 composed of about 15 microns of silicon thermal oxideis shown over a silicon substrate 12.

The compound resonator 16 acts on an optical input signal through atransfer function to affect the optical properties of the output signalsuch as center wavelength, amplitude, phase, and shape. For an opticalfilter type device, the properties of interest are generally passbandshape and center wavelength. These properties of the output signal areoptimized through a calibration process. The calibration processcomprises the steps of measuring the optical signal properties ofinterest and adjusting the local temperatures of each of the opticalresonators 30 a, 30 b, and 30 c to obtain optimum optical propertyvalues. The local temperature of each optical resonator 30 a, 30 b, and30 c is adjusted by changing the power dissipation in the associatedresonator heater 32 a, 32 b, and 32 c. The result is the generation of acalibration data point for each set of desired optical properties of theoutput signal. This calibration data point is characterized by themeasured value of the compound optical resonator temperature sensor 22and the power dissipation in each of the optical resonator heaters 32.

FIG. 7 shows a block diagram example of a control system 116 forregulating a 3-ring compound optical resonator 82. The individualheaters 32 a, 32 b, and 32 c localized to each optical resonator 30 a,30 b, and 30 c are denoted by 32 a, 32 b, and 32 c, and the temperaturesensor localized to the compound resonator cavity is labeled 22. Theoutput of the measurement circuit 118 that is monitoring the value ofthe temperature sensor 22 is connected to a heater controller 120. Thecombination of sensor and controller make a feedback circuit. Thecontroller 120 adjusts or regulates the power dissipation in one of theheaters 32 (in this case, the center ring heater 32 b) to maintain ormove the temperature sensor reading towards a desired set point thatmay, for example, correspond to a specific resonant wavelength. Thisrelationship between sensor reading and filter wavelength would havebeen determined during an earlier calibration or optical measurementprocess.

The power dissipated in the remaining resonator heaters 32 is regulatedby controllers 122 having as their input the value of the powerdissipation output of controller 120. These other two heaters 32 a and32 c are slaved to the dissipation power of the heater 32 b under directcontrol of the feedback loop. The power dissipation for these resonatorheaters 32 is calculated and set according to a predetermined functionbased and derived from the calibration data points. The function inputvariable is the power dissipated in the heater 32 b under control of thefeedback loop.

The invention is not limited to the example of a 3-ring compoundresonator 82 and may comprise N coupled optical resonators. As anexample, one resonator heater 32 b would be under control of thefeedback loop whereas the other (N−1) heaters 32 a or 32 c would beslaved to the power dissipation in the one resonator heater 32 baccording to any number of functional relationships, from 1 to (N−1)distinct functions. It is also worth noting here that an additional andmore global control system feedback loop may be added to the overallintegrated optical device 10. Since the localized temperature sensor 22is not directly contacting the waveguide material but rather spaced asmall distance away, there is a possibility for small temperatureoffsets or errors due to global temperature changes in the environmentor substrate 12. For devices requiring very precise temperature controlof less than 1 degree Celsius, it is preferred that the substrate orwafer 12 temperature be regulated as well. This may be done with acontrol system comprising a thermoelectric cooler (TEC) or heater inthermal contact with the substrate 12, an associated controller ordriver circuit, and a temperature sensor 22 not localized to anyspecific thermo-optic device but rather in thermal contact with thesubstrate itself.

FIG. 8 shows an example output signal from a 3-ring compound opticalresonator 82. The compound optical resonator 82 is acting as a tunablefilter element that is passing a band of optical frequencies. The filterpassband data is plotted as a normalized optical intensity versusfrequency, with the center frequency set to zero. The dashed line showsthe filter shape with zero power dissipation in all the opticalresonator heaters 32. The filter shape deviates from an ideal flatpassband due either to a variation in refractive index or a detuning ofthe optical resonator ring radii from the optimal design value. Acalibration process can be used to adjust the power dissipation in eachheater 32 a, 32 b, and 32 c to achieve the desired filter shape, whichin this case is a flat passband, and the resulting data is shown by thesolid line. FIG. 9 shows a set of optimized filter shapes from the3-ring compound resonator element 82. Each passband corresponds to adifferent temperature sensor set point that is being used to regulatethe power dissipation of the middle optical ring resonator heater 32 b.The outer two optical resonator heater (32 a and 32 c) powerdissipations are set by a function determined by the calibrationprocess.

FIG. 10 shows an example set of calibration data for one of the outertwo rings in relation to the power dissipation of the center ring, whichis under control of the feedback loop. The pulse width modulation (PWM)duty cycles for the controllers 120 and 122 a of each heater is plottedin arbitrary units and corresponds to power dissipation. In this case,the power dissipations in the outer ring heaters 32 a and 32 c wasoptimized during a calibration process for seven center ring powerdissipation values. Using this data, a functional relationship can begenerated that will determine an outer ring heater power dissipationvalue for any center ring power dissipation value that is regulated bythe feedback loop. The relationship may be a simple linear relationshipas shown, a second or third order polynomial, or any other functionalform that fits the data set well.

In use, the compound resonator 16 acts on an input signal through atransfer function to manipulate the optical properties of the outputsignal such as center wavelength, amplitude, phase, and shape. Theoutput optical signal properties of a compound optical resonator 16 areoptimized through a calibration process comprising the steps of:measuring the optical signal properties of interest and adjusting thelocal temperatures of each of the optical resonators 30 a 30 b, and 30 cto obtain optimum values. The local temperature of each opticalresonator 30 a, 30 b, and 30 c is adjusted by changing the powerdissipation in the associated resonator heater 32 a, 32 b, and 32 c. Acalibration data point is generated for each set of desired opticalproperties for the output signal from the compound optical resonator 16.This calibration data point is characterized by the measured value ofthe compound optical resonator temperature sensor 22 and the powerdissipation in each of the optical resonators 30 a, 30 b, and 30 c.

The desired optical properties are maintained or reproduced at a latertime using a feedback loop. The feedback loop is used by the controlsystem 116 to directly control or adjust the power dissipated in atleast one of the localized heaters 32 a, 32 b, or 32 c. The output ofthe feedback loop is based on a loop input value that is the measuredvalue of the temperature sensor 22 localized to the compound opticalresonator 16, and the power is adjusted in order to achieve the desiredtemperature sensor value. The power dissipated in the remainingresonator heaters 32 is slaved to the dissipation power of the heater orheaters 32 under direct control by the feedback loop. The powerdissipation in the remaining resonator heaters 32 is calculated and setaccording to a predetermined function based and derived from thecalibration data points. The functional input variable is the powerdissipated in the resonant heater 32 under control of the feedback loop.

In one preferred embodiment, the present invention overcomes thelimitations of existing and proposed thermal control systems forcompound optical resonators. By utilizing only one temperature sensor 22per compound optical resonator 16, the number of feedback loops inoperation for a given integrated optical device 10 are reduced. Whereasthe integrated optical device 10 having four compound optical resonators16, each containing five ring resonators 84, heaters 32, and temperaturesensors 22 requires twenty active feedback loops for operation, thepresent invention reduces the required number to five. The result is alarge reduction in the number of electrical leads on the optical chip aswell as a corresponding reduction in the complexity of the electronicshardware and software driving the integrated optical device.

For example, electronic designs often rely on intermittent measurementof process variables in order to temporally multiplex or reuse a portionof an electrical circuit. Typically, a process input variable, such asan RTD value, of a feedback loop is measured and an adjustment is madeto the output variable, which is power dissipation in this case. Thisparticular feedback loop is then left alone while other processvariables are measured and adjustments are made. The electronic circuitperiodically returns to the original feedback loop to measure andadjust. By reducing the number of feedback loops in operation, a givenelectronic circuit design can measure and adjust more often.

A second significant advantage of having only one feedback loop for eachcompound optical resonator 16 is improved stability. Compound opticalresonators having multiple feedback loops are effected by thermalcrosstalk between the temperature sensor 22 of one resonator 30 and theheaters 32 of adjacent resonators 30. For example, as one heater controlloop adjusts power upwards in response to a measurement from itsfeedback temperature sensor 22, the feedback temperature sensors 22 of aneighboring resonator 30 will record a portion of the temporarytemperature rise. In this manner, independent control loops having largethermal crosstalk can oscillate around a set point or simply becomeunstable or noisy. It becomes exceedingly difficult to tune theresonator control systems for higher order compound resonatorscontaining three or more rings. Extensive damping could be added to thecontrol system 116 to mitigate some of the effects of thermal crosstalk,however, there is a penalty in terms of the speed of response of thecontrol loop to changes in setpoint.

One more advantage of the integrated optical device 10 of the presentinvention is the ability to modify the transfer function of the compoundoptical resonator 16 by applying different temperatures to the differentoptical resonators 30 making up the compound resonator 16. Often in themanufacturing process of fabricating integrated optical devices, thereare slight variations in refractive index or waveguide dimensions forthe different optical resonators. For example, a tunable optical filterconstructed from compound resonators having three rings resonators willhave an optimum flat-top passband only if the resonant wavelengths ofall three rings are similar. In fact, for an optical filter having aring resonator design 64 with a radius of curvature of about 50 microns,a deviation in radius between two rings of only one or two nanometersdetunes the resonant wavelengths by enough to degrade the filter shape.The result is a very tight requirement on the manufacturing process. Theintegrated optical device 10 allows detunings of the various opticalresonators 30 making up a compound resonator 16 to be corrected byadjusting the local temperature. A ring 64 having a slightly lowerresonant wavelength than desired can be heated to match the resonancesof the other rings 64 in the compound resonator 16. The additionalheating requirement can simply be incorporated into the predeterminedfunction during the calibration process that determines the powerdissipation of that ring 64.

It will be understood from the foregoing description that variousmodifications and changes may be made in the preferred and alternativeembodiments of the present invention without departing from its truespirit.

This description is intended for purposes of illustration only andshould not be construed in a limiting sense. The scope of this inventionshould be determined only by the language of the claims that follow. Theterm “comprising” within the claims is intended to mean “including atleast” such that the recited listing of elements in a claim are an opengroup. “A,” “an” and other singular terms are intended to include theplural forms thereof unless specifically excluded.

1. A device comprising: a substrate; a first and a second opticalwaveguide provided on the substrate; a compound optical resonatorprovided on the substrate, the compound resonator optically coupled tothe first and second waveguides, the compound optical resonatorincluding a first optical resonator and a second optical resonatorcoupled to the first optical resonator; a first heater configured toadjust a temperature of the first optical resonator; a second heaterconfigured to adjust a temperature of the second optical resonator; anda control system configured to sense a temperature of the compoundresonator and control a first power dissipated by the first heater inresponse to the sensed temperature of the compound resonator, andcontrol a second power dissipated by the second heater based on thefirst power.
 2. The device of claim 1, wherein the control systemincludes a temperature sensor configured to sense the temperature of thecompound resonator.
 3. The device of claim 2, wherein the temperaturesensor comprises a metal film resistor.
 4. The device of claim 3,wherein the temperature sensor has a resistance between about 100 ohmsand 10000 ohms.
 5. The device of claim 2, wherein the temperature sensorincludes a thermistor.
 6. The device of claim 4, wherein the controlsystem includes a control loop, such that the control system is furtherconfigured to control the first power.
 7. The device of claim 1, whereinthe first heater is a first of a first plurality of heaters and thesecond heater is a first of a second plurality of heaters and the secondpower is a first of a plurality of second powers, the first powerdissipated by the first of the first plurality of heaters being adjustedbased on the temperature of the compound optical resonator, each of theplurality of second powers dissipated in a corresponding one of thesecond plurality of heaters being adjusted based on the first power. 8.The device of claim 7, wherein one of the plurality of second powersdissipated in a corresponding one of the second plurality of heaters isa linear function of the second power.
 9. The device of claim 7, whereinone of the plurality of second powers dissipated in a corresponding oneof the second plurality of heaters is a polynomial function of thesecond power.
 10. The device of claim 7, wherein one of the plurality ofsecond powers dissipated in a corresponding one of the second pluralityof heaters is a second order polynomial function of the second power.11. The device of claim 7, wherein an electrical power is applied toeach of the first and second pluralities of heaters, the electricalpower being pulse-width modulated.
 12. The device of claim 1, whereinthe first heater is a first resistive heater and the second heater is asecond resistive heater.
 13. The device of claim 12, wherein the firstand second resistive heaters include metal film.
 14. The device of claim13, wherein the metal film comprises one or more elements selected froma group consisting of platinum, gold, nickel, chrome, aluminum, andtungsten.
 15. The device of claim 12, wherein the resistance of thefirst and second heaters is between about 10 ohms and 500 ohms.
 16. Thedevice of claim 1, wherein the first heater comprises a Peltierthermo-electric element.
 17. The device of claim 1, wherein the secondheater comprises a Peltier thermo-electric element.